Memory performance, hippocampal volumes and memory fMRI in frontal versus temporal lobe epilepsy

Patients with temporal lobe epilepsy (TLE) and patients with frontal lobe epilepsy (FLE) show an increased risk of memory impairment relative to healthy controls, alongside structural and functional encoding-related cerebral alterations. To better specify syndrome-related differences, we directly compared TLE and FLE patients.

We evaluated 56 TLE (25 right-sided) and 24 FLE patients with respect to fMRI activation during encoding of scenes, faces, and words, post-scan recognition accuracy, neuropsychological verbal and figural memory tests, and hippocampal volumes.

In the fMRI recognition paradigm, FLE patients were similarly impaired as both left- and right-sided TLE patients for scenes, faces and words. In contrast, on neuropsychological tasks involving multiple learning trials, we found a pattern of worse verbal recognition and delayed recall in left-sided TLE patients than in FLE patients, whereas right-sided TLE patients performed worse on figural recall. TLE patients also had smaller hippocampal volumes than FLE patients on the side of their seizure onset. For encoding-related hemodynamic activity, left-sided TLE patients showed lower left mesio-temporal lobe (mTL) activation during scene and face encoding than FLE patients. No group differences emerged in mTL activation during word encoding; however, FLE patients showed in tendency stronger right frontal activation than right-sided TLE patients. In addition, right-sided TLE patients showed weaker deactivation of the left middle temporal gyrus than FLE patients during face encoding. Default mode network analyses likewise indicated that weaker deactivation during the task is more marked in TLE than FLE patients.

These findings reveal similarities but also underline task- and material-specific differences in memory performance and functional integrity between TLE and FLE.